(1) Field of the Invention
The present invention relates to an improved method for recovering ammonia from spent ammonia-base sulfite pulping liquor. Specifically, this invention is directed to a process whereby ammonia is recovered from spent ammonia-based pulping liquor and converted into a concentrated cooking liquor comprising ammonium bisulfite and ammonium sulfite for use in the digester of a pulping plant. Accordingly, the general objects of the present invention are to provide novel and improved methods of such character.
(2) Description of the Prior Art
One method of producing pulp, for papermaking, involves dissolving the ligneous and other non-celluosic substances of the wood with acidic or neutral liquors. Those liquors are generally comprised of sulfite and a base element such as calcium, magnesia, sodium or ammonia. The use of ammonia as the basic element in the "cooking liquor" has the advantage of producing a shorter cooking time in comparison with the use of calcium as the basic element. Ammonia-base sulfite pulping is also advantageous since it is the least capital intensive pulping process and is the most acceptable from an environmental standpoint. However, ammonia is a comparatively expensive raw material and it is difficult to recover the ammonia from the spent liquor. A further disadvantage inherent in the use of ammonia is the necessity of preventing the discharge thereof into the environment; this problem also dictating that the ammonia be recovered from the spent cooking liquor.
U.S. Pat. No. 3,864,192 to Copeland reveals a typical prior art process for the treatment of spent ammonia-base pulping liquor. In this patent the spent liquor is converted, in part or entirely, to a magnesium-base sulfite waste liquor by mixing ammonia-base liquor with hot gases, at about 1600.degree. F., containing magnesia particles. The resulting magnesia-base bisulfite waste is then concentrated and burned to produce the magnesia particles. In the process of U.S. Pat. No. 3,864,192, sulfur dioxide but not ammonia is recovered and very high temperatures must be maintained in order to produce the magnesia particles.
U.S. Pat. No. 3,598,695 to Waterstradt purports to disclose a prior art technique for recovering ammonia from ammonia-base pulping liquor. In the process of this patent the ammonia-base spent liquor is first combined with sulfuric acid to effectuate the release of sulfur dioxide. Subsequently, a sodium hydroxide solution is added to the spent liquor to effectuate the release of ammonia. A principle disadvantage of the technique of U.S. Pat. No. 3,598,695 is the requirement for the use of two reagents; i.e., sulfuric acid and sodium hydroxide; to serially effectuate the release of sulfur dioxide and ammonia.